The new test is based around PAP smears of the type normally taken for cancer screening. Cells from the foetus are isolated, genetically identified, and screened for a wide range of abnormalities such as Down syndrome and cystic fibrosis.
Current prenatal tests, such as amniocentesis and chorionic villus sampling, involve putting a needle into the womb to obtain fluid. This carries with it about a one per cent chance of miscarriage. The risk of miscarriage and the high cost of testing mean that these invasive techniques are offered only to mothers over 35 or those at high risk. And only one in 20 indicate an affected pregnancy.
Because only older women are tested, about 80% of Down syndrome pregnancies in younger mothers in Australia are currently not identified. The new prenatal test can safely be offered to all pregnant women. It also ensures that invasive tests, with the associated miscarriage risk, are only performed when necessary for confirmation.
A further advantage of the new test is that it can be easily performed by general practitioners. The samples can then be sent to laboratories through the post for analysis, allowing greater access to prenatal testing in rural Australia and worldwide.
The study which led to the new test, undertaken in Brisbane hospitals, collected PAP samples from 600 women who were between 5 and 35 weeks pregnant. The research team now hopes to perform testing much earlier in pregnancy, giving couples more time to make informed choices. Current amniocentesis testing is performed between 16 and 20 weeks of pregnancy with results taking 2 to 3 weeks. The researchers hope this new PAP technique can be performed as early as 5 weeks with results being available the same day. The new test is also likely to be less costly than conventional techniques which should increase the accessibility of prenatal testing for all.
"For the first time prenatal testing can be offered to all women," Mr Irwin said. He is currently completing his PhD at the University of Queensland with Dr Findlay.
While foetal cells have been identified in the cervix for many years, this is the first time they have been efficiently isolated from PAP smears. These cells are then DNA fingerprinted using PCR techniques to confirm their foetal origin unequivocably. Genetic diagnosis can be undertaken in the same test. PCR and SNP techniques which can identify tiny changes in the genetic code allow a much wider range of tests to be performed than conventional prenatal techniques. Clinical trials are progressing and it is hoped that this test will be available within 2 or 3 years.
For further information, contact Mr Darryl Irwin on 0412 779 528 or Dr Ian Findlay on 0402 979 983
USING SINGLE CELL DNA FINGERPRINTING TO IDENTIFY FETAL CELLS SERIALLY ENRICHED FROM PAP SMEARS.
DARRYL IRWIN1, Ian Findlay 1,2
1. Institute for Molecular Biosciences, University of Queensland, Brisbane, Australia.
2. Australian Genome Research Facility, The University of Queensland, Brisbane, Australia.
Introduction
Prenatal genetic diagnosis of chromosomal and single gene disorders currently requires the extraction of fetal cells from the uterine cavity by invasive amniocentesis or chorionic villus sampling (CVS). Although highly reliable, they have procedurally related risks such as miscarriage, require a high level of technical expertise and are performed relatively late in pregnancy.
Therefore there has been a strong motivation for many years to develop minimally invasive techniques, ideally by isolation of fetal cells from the endocervical canal or maternal blood. Although these techniques have diagnostic potential they have been severely hindered by major limitations in cell isolation, identification and diagnostic techniques used. Multiplex fluorescent PCR (MFPCR) can overcome many of these limitations by simultaneously DNA fingerprinting, sexing, assaying for single gene defects and diagnosing chromosomal aneuploidy from a single cell within a single reaction. DNA fingerprinting also has the advantage of being highly discriminating for cell origin even when applied to close relatives such as mother and offspring.
Method
In this study we demonstrate the efficiency of serially enriching fetal cells from pap smears and secondly the identification of such cells as fetal using single cell DNA fingerprinting.
Results
After enrichment the percentage of enriched fetal cells were identified using single cell DNA fingerprinting at a final average population of 37% fetal cells, facilitating efficient isolation of single fetal cells from 90% of samples analyzed.
Conclusion
For the first time, absolute identification of such fetal cells is achievable allowing non-invasive prenatal diagnosis to become a reality.